The present invention relates to improved cost-effective ultraviolet optical filtering devices. More particularly, the present invention relates to selectively tuned ultraviolet optical filters useful in mercury vapor lamp based UV water purification systems.
Purified water is essential not only for drinking purposes, but also for numerous other applications such as, for example, drug and food manufacturing, semiconductor processing, critical cleaning applications, heat exchanger coolant use, purification of swimming pool water, etc.
Of particular concern in the water purification industry is that of providing purified drinking water to third world countries. Significant challenges continue to exist in this area due to the need for a high degree of purification and utmost reliability required to prevent water-borne diseases (cholera, typhoid, hepatitis, etc.). In addition, such water purification treatment processes must have the capacity to produce high volumes of purified water at the lowest possible cost.
Chemical, biological and physical treatment processes are well-known and are capable of providing water of varying degrees of purity. One popular process though is the exposure of germ-laden water to the germicidal wavelength of an ultraviolet source. Exposing flowing water to the ultraviolet germicidal wavelengths of 200-300 nm alters and damages a bacteria""s DNA, thereby preventing its reproduction. DNA absorbs ultraviolet light strongly in the ultraviolet spectrum centered at 260 nm. Thus, the typical dominant 254 nm emission of a mercury vapor lamp has been employed for this purpose.
The U.S. Public Health Service requires that ultraviolet water purification equipment have a minimum 254 nm ultraviolet dosage of 16,000 micro-watt-seconds per square centimeter. In order to insure that this minimum criteria is satisfied, an ultraviolet mercury lamp is often monitored with an optically filtered silicon photosensor (which directly measures the 254 nm emission). If the silicon photosensor measures a low 254 nm emission, a warning is activated to replace the substandard ultraviolet lamp. Determining when an ultraviolet mercury lamp has aged to the point where its germicidal effectiveness is diminished is critical.
Some existing expensive UV water purification systems are in use which employ ultraviolet enhanced photodiodes fitted with standard optical bandpass filters to monitor the life of the mercury lamp. These optical bandpass filters define the performance of the optical system by selecting the critical 254 nm emission, while optically blocking the remaining full UV/VIS/IR spectral region (200 nm to 1200 nm). Although successful in their application, such standard optical filters are very expensive, are limited in their 254 nm performance, and have substandard durability which limits their longevity, field lifetime and versatility. Further, these optical bandpass filters have poor resistance to environmental exposure (e.g. moisture and temperature) and, thus, need to be very carefully hermetically sealed within the housing of the photosensor. Thus, such optical filters are not suitable in applications, such as water purification in third world countries, which require utmost reliability at the lowest possible cost.
Presently available optical filters used in water purification systems are expensive standard narrow bandpass filters centered at 254 nm. The two general types are: MDM (Metal-Dielectric-Metal) filters and Solar Blind Filters.
MDM filters consist of transparent quartz (or similar) substrates optically coated with alternating thin films of a soft dielectric (e.g. cryolite) and metallic aluminum. Disadvantages of MDM filters include: poor resistance to elevated temperature, extreme fragility (soft, easily scratched optical coatings limits their use) which requires the coatings to be protected with additional quartz substrates, thickness and size constraints and extreme cost (approximately $88 per filter). Shown in FIG. 4 is the spectral behavior of a typical 254 nm MDM bandpass filter. Solar Blind Filters are multi-element devices manufactured with absorptive glasses and optical crystals (e.g. nickel sulfate). Such filters are very sensitive to moisture and heat, are very thick (5-6 mm) and cost prohibitive (approximately $250 per filter). Shown in FIG. 5 is the spectral behavior of a standard Solar Blind Filter. Both MDM and Solar Blind Filters are limited in their application because they must be mounted and sealed within a photodiode housing. Because of manufacturability, the filter sizes must be large and cover the full clear aperture of the photodiode housing (see FIG. 3).
It would, thus, be desirable to provide improved optical filters for ultraviolet water purification systems that are capable of producing large volumes of highly purified water with utmost reliability and at the lowest possible cost.
The present invention provides an improved method and apparatus for the ultraviolet purification of liquids, particularly water, at a substantially lower cost and greater reliability than currently available systems. More particularly, the present invention provides unique ultraviolet optical filters selectively tuned to eliminate the discrete non-germicidal wavelength polychromatic background emissions from mercury lamps.
In addition to the typical 254 nm emission of a mercury lamp, mercury lamps also have polychromatic background emissions at other discrete wavelengths, e.g., 313 nm, 365 nm, 405 nm, 436 nm, 546 nm, 579 nm, 1015 nm and 1140 nm (see FIG. 2). This is a unique characteristic of these types of lamps. These wavelengths do not contribute to water purification, but do interfere with the accurate optical monitoring of the mercury lamp life using a silicon photodiode. To monitor the critical 254 nm mercury lamp emission, these wavelengths must be blocked. Whereas standard expensive MDM or Solar Blind 254 nm bandpass filters fully block the entire UV/VIS/IR spectral regions, it is the unique and novel feature of this invention to block only these discrete background wavelengths. In this way, the optical filter cost is dramatically reduced and the reliability improved.
The optical filters of the present invention provide a number of advantages over prior optical filters including, for example, very low cost (less than $2 as opposed to $88 for MDM filters and $250 for Solar Blind Filters), extreme durability to high temperatures and moisture, superior scratch resistance, small sizes, improved optical performance, extended physical longevity, high imaging quality of transmitted radiation, and improved throughput of the transmitted critical 254 nm wavelength. Further, preferred filters of the invention do not require any sealing from the ambient atmosphere and do not degrade over time with exposure to ultraviolet irradiation.
The optical filters of the present invention may be fabricated by conventional optical coating technologies including, for example, physical vapor deposition (thermal evaporation employing electron-beam technology), ion assisted deposition, ion beam or magnetron sputtering, chemical vapor deposition or reactive ion plating.
The design and dimensions of the optical filters in accordance with the present invention makes them particularly suitable for use in water purification systems that employ ultraviolet enhanced photodiodes fitted with optical filters. In one preferred embodiment, the optical filters of the present invention comprise a substrate having optical coatings thereon. Such optical filters may suitably form the external window of the photodiode. In a particularly preferred embodiment, optical coatings are directly deposited upon the photodiode surface itself, which provides particularly substantial cost savings (FIG. 1).
In accordance with one embodiment of the present invention, the optical filter comprises a substrate with optical coatings deposited on one or both surfaces of the substrate. The substrate may be selected from a wide variety of conventional optical substrates including, for example, glass, plastic, fused silica, metal or the like. In preferred embodiments, the substrate is either a single thin fused silica substrate or an ultraviolet transparent glass substrate.
The coating material of the present invention may be a variety of materials recognized by those skilled in the art including low and high refractive index materials. Low refractive index (nL) materials include, for example, SiO2, Al2O3, SiO, fluorides such as barium fluoride and lanthanum fluoride, MgO, etc. Collectively, low-index materials are sometimes referred to herein as xe2x80x9cLxe2x80x9d. Low-index materials (L) are defined to mean herein materials having a refractive index (20/D) of less than 2.0, more typically 1.8 or less such as 1.8 to 1.3. Common high refractive index (nH) materials include, for example, TiO2, ZrO2, Ta2O5, and HfO2. Collectively, these high-index materials are sometimes referred to herein as xe2x80x9cHxe2x80x9d. High-index materials (H) are defined to mean herein materials having a refractive index (20/D) of 2.0 or greater. As known to those skilled in the art, the designation xe2x80x9c20/Dxe2x80x9d indicates the refractive index values are as measured at 20xc2x0 C. using a light source of the D line of sodium. In particularly preferred embodiments, the coating materials are thin films of ultraviolet transparent refractory metal oxide (e.g. hafnium oxide, zirconium oxide, silicon dioxide, etc.).
In some embodiments, rather than form the optical coatings on the above-described substrate materials, the optical coatings may, if desired, be coated directly upon the photodiode active area.
The spectral design of the optical filter is tuned specifically in accordance with its ultimate use. For ultraviolet water purification, the optical filter transmits effectively within the wavelengths that contribute to ultraviolet sterilization (centered at 254 nm) and selectively rejects those background discrete wavelengths in the UV/VIS/IR emission spectra of typical mercury lamps and which fall within the sensitivity region of photodiodes.
The specially tuned optical filters of the present invention, when used in connection with ultraviolet enhanced photodiodes, provide accurate low-cost monitoring of the critical 254 nm emission required in water purification procedures.
Other aspects of the invention are discussed infra.